1. Field of Invention
The invention relates generally to a Global Navigation Satellite System (GNSS) augmentation system and, in particular, to differential methods and apparatus to support aircraft landing operations.
2. Description of Related Art
The Navstar Global Positioning System (GPS) is an all-weather, worldwide, highly accurate three dimensional radionavigation system. Developed and operated by the U.S. Department of Defense (DOD), the system includes a constellation of Earth-orbiting satellites at semisynchronous altitude, a dedicated ground control system which supports those satellites, and a large number of user terminals ("GPS receivers") which passively track the satellite broadcast L-band signals (1.6 and optionally 1.2 GHz) in order to precisely determine their position and velocity in the World Geodetic System 1984 (WGS-84) coordinate frame along with internal GPS system time which is, in turn, related to Coordinated Universal Time (UTC).
GPS is just one example of a GNSS. Another example is the Global Orbiting Navigation Satellite System (GLONASS) operated by the Russian Federation. GPS and GLONASS can each function as a stand-alone GNSS, or they may be used together in a hybrid GNSS manner with combined GPS/GLONASS receivers passively tracking the L-band signals broadcast by both sets of satellites for improved accuracy and reliability. In the future, the on-orbit GPS and GLONASS satellites will be joined by other navigation satellites developed and operated by civil agencies, notably the U.S. Federal Aviation Administration (FAA), which are specifically designed to be used in conjunction with GPS or GLONASS or both GPS and GLONASS as part of an overarching GNSS for even better accuracy and reliability.
Although having more satellites available for use in a GNSS, GPS improves the accuracy and reliability of that GNSS, particularly with respect to improving the availability of integrity (i.e., the ability of the GNSS receiver to verify the accuracy of its position and velocity solution). However, there are economic limits as to how many satellites can be placed in orbit and hence, limits as to how accurate and reliable a GNSS can be. For applications that demand high levels of accuracy, reliability, and integrity, the typical method of achieving these levels for a GNSS is to employ correction signals from a ground-based differential system to augment the satellite signals being tracked by the GNSS receiver. These augmentation signals from the ground-based system are intended to differentially correct for those aberrations of the satellite signals which will be tracked by the GNSS receiver and will, if not differentially corrected for, cause the GNSS receiver to compute a slightly erroneous position and velocity solution. As long as the differential system itself operates with very high accuracy, reliability, and integrity, the augmentation signal corrections provided to the GNSS receiver can counteract the effects of satellite signal aberrations which are external to the GNSS receiver, leaving the GNSS receiver internal effects (tracking inaccuracies, software errors, hardware faults, and the like) as the limiting factor in accuracy, reliability, and integrity.
Presently, the most widely used ground-based differential system for GNSS is a stand-alone Differential Global Positioning System (DGPS) ground station. Traditionally, DGPS ground stations have used a "reference and monitor" architecture to maintain the accuracy and integrity of their output differential correction signals. This is basically the same architecture used by many existing ground-based radionavigation aids such as instrument landing systems (ILSs) and microwave landing systems (MLSs). The design of a traditional DGPS ground station incorporates two independent GPS receivers, each with its own dedicated receiving antenna, which are installed at separate locations with well-known coordinates. One of the receivers is the source reference receiver and the other receiver is the integrity monitor receiver. Under this traditional design, the source reference receiver tracks the incoming satellite signals, uses its known coordinates to determine the effect of any satellite signal aberrations, develops mathematical correction factors which will counteract the effect of the satellite signal aberrations, and broadcasts those corrections to the mobile GPS receivers that are specially equipped to receive, process, and apply the differential correction factors to their own satellite signal tracking measurements. The integrity monitor receiver serves only to verify the integrity of the differential correction factors broadcast by the source reference receiver either by 1) mimicking a mobile GPS receiver and applying the broadcast corrections to its own measurements to verify that the resulting position and velocity closely matches the known coordinates and known velocity (zero) of the integrity monitor receiver, or 2) mimicking the source reference receiver and verifying that the differential correction factors it computes using its own measurements and known coordinates closely match the corrections broadcast by the source reference receiver.
Although a traditionally designed DGPS ground station is sufficient for many applications, it is wasteful in terms of accuracy and integrity. Because correction data from only one receiver (the source reference receiver) is broadcast to the mobile GPS receivers, while the other receiver (the integrity monitor receiver) simply verifies the broadcast correction data, no significant advantage is gained from having two receivers in the DGPS ground station. The delivered accuracy of the DGPS ground station is entirely dependent on the accuracy of the source reference receiver, while the delivered integrity is entirely dependent on the integrity of the integrity monitor receiver. The maximum advantage of having two (or more) receivers in the DGPS ground station can only be realized if both (or all) receivers contribute to the delivered accuracy and integrity of the broadcast correction data.
The qualities of accuracy and integrity are together known as "veracity". Regardless of the GNSS satellites being used, a veracious differential GNSS (DGNSS) ground station broadcasts differential corrections which are corrupted by a lower level of errors than the corresponding differential corrections from a non-veracious DGNSS ground station and--just as importantly--also broadcasts data describing the level of errors corrupting the differential corrections which are more reliable than the corresponding data from a non-veracious DGNSS ground station. For aircraft landing operations, the critical differential correction data which must be veracious are the pseudorange correction (PRc) values for each GNSS satellite signal and the user differential range error (UDRE) values for each PRc value. Although there is only one PRc value for each GNSS satellite signal at any one time, veracious DGNSS ground stations provide at least two different UDRE values for each PRc value. One UDRE value (SIGMA) describes the level of noise errors corrupting the associated PRc value while the other UDRE value (BIAS) describes the total bias error corrupting that PRc value. Veracious DGNSS ground stations broadcast accurate PRc values along with reliable SIGMA and BIAS UDRE values to provide the integrity for each PRc value.
Industry currently lacks devices necessary for efficient and veracious DGNSS ground stations. Some DGNSS ground stations are efficient and some DGNSS ground stations are veracious, but there are no DGNSS ground stations are both efficient and veracious.